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THE ACID BESSEMER PROCESS.                           HI
Ehrenwerth* argues that pig-iron low in silicon should give better steel, for, with high silicon., there is a greater proportion of free oxygen, in the gases during the first stages of the blow. The percentage of carbon is nearly constant in all irons, and, with an. increase in silicon, there is a corresponding increase in the proportion which the silicon bears to carbon. Granting that the presence of free oxygen in the gases escaping from the vessel during the first part of the process is due to the proportionately greater quantity of silicon as compared with carbon, then if the metal at the end of the operation should contain a high proportion of silicon as compared with its content of carbon, the escaping gases would contain free oxygen. This proportionately high silicon at the end of the operation is found in heats which contained a high initial percentage of silicon in the iron, and hence such heats would be expected to have free oxygen in the bases which are formed at the close of the operation, and this" free oxygen will signify a more highly oxidized condition of the metal.
Notwithstanding that tipping the converter has rendered unnecessary as large a margin of calorific power as was formerly necessary, it is advantageous to have a slight excess of silicon to allow for delays and new bottoms, so that it is necessary to lower the temperature of normal charges by the addition of steel scrap or solid pig-iron. The skill attained in estimating the temperature-of melted steel seems almost incredible to the lay mind, for it is possible to detect the difference caused by a variation of 100 pounds in the amount of scrap added to a 7-ton charge in the converter, and I have else-wheret tried to show that this represents a difference of only 13 C. It must be acknowledged that all heats are not regulated to such exact measure, but a variation of three or four times this amount is more than is expected in current American practice. This accuracy can only be obtained by uninterrupted work, so that we find that the best "scrapping" follows the fastest running. This fact is an answer to the criticism of foreign metallurgists that the large outputs of American Bessemer plants have been made at the expense of quality. There is no evidence to show that an ample supply of air, and a shorter blow, will give an inferior product, but, on
* Da* Berg- und Huttenwesen avf der Wettausstellung in Chicago.   Ehrenwerth, 1895, p. 276. t The Open-Hearth Process,   Trans, A, I.M.E.< Vol. XXII. p, 392.